1. Field of the Invention
The present invention relates to a reproduction method and a reproduction apparatus for speedily reproducing MPEG-compressed video signals recorded on a recording medium.
2. Description of the Related Art
MPEG (Moving Picture Experts Group) compression is known as a compression encoding method for performing the inter-frame compression of video signals. MPEG-compressed signals are composed of GOPs (Group of Pictures), each composed of an I picture generated by intra-frame or intra-field compression, P pictures generated by forward predictive compression, and B pictures generated by bi-directional predictive compression. Each GOP contains one I picture.
When reproducing video signals, at a speed higher than the regular speed, from a recording medium on which MPEG-compressed video signals are recorded, it is difficult to decompress the I picture, all P pictures, and all B pictures because of low processing speed. Therefore, during high-speed reproduction, only part of pictures are decompressed.
FIGS. 1A to 1D show how pictures are output when video signals are reproduced, at a speed higher than the regular speed, from a recording medium on which MPEG-compressed video signals are recorded. In these figures, the vertical axis indicates picture numbers and the horizontal axis indicates the elapsed time. The actual output pictures are indicated by bars, while the ideal output pictures for smooth, high-speed reproduction are jointed by straight lines.
A GOP of MPEG-compressed signals to be reproduced, which is used in the description below, is composed of a total of 15 pictures including one I picture, four P pictures, and ten B pictures. They are in order of I1, B2, B3, P4, B5, B6, P7, B8, B9, P10, B1, B12, P13, B14, and B15.
Let M be the number of pictures of a GOP, and let N be the number of pictures from a P picture (this P picture not included) to the next P picture or I picture. When the speed multiplier is N or smaller, high-speed reproduction video is generated using I pictures and P pictures. On the other hand, when the speed multiplier is larger than N, high-speed reproduction video is generated using only I pictures.
That is, when the speed is 1.2× or 2× as shown in FIGS. 1A and 1B, the speed multiplier is smaller than 3 and therefore high-speed reproduction video is generated using I pictures and P pictures. On the other hand, when the speed is 4× or 8×, the speed multiplier is larger than 3 and therefore high-speed reproduction video is generated using only I pictures. This reproduction method applies not only to the case when M=15 and N=3 described above but also to other cases. When the speed multiplier is smaller than N, high-speed reproduction video is generated using I pictures and P pictures. On the other hand, when the speed multiplier is larger than N, high-speed reproduction video is generated using only I pictures.